Both he and his wife are half 6th cousins, three times removed, of mine, he by the Powys connection, she by the Lybbe connection.

Possibly his birth registration in Ireland:

First name(s) Thomas
Last name Pakenham
Registration year 1864
Registered quarter/year 1864
Registration district Dublin South
Volume 17
Page 730
County Dublin
Country Ireland
Record set Irish Births 1864-1958

I think this corroborates with CP’s statement that he was born as the second twin on 19 Oct 1864 at Leinster St, Dublin.  His elder brother died aged 12.
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In 1881 he was a pupil at Winchester, aged 16 and born in Dublin, Ireland.

In 1911 he was a colonel in the army on half-pay and a landowner, living at Penn House, Penn, Bucks with his wife, 2 s. and 2 daus.  He gave his name as Thomas Pakenham, Earl of Longford.
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From CWGC site:

PAKENHAM, THOMAS

Rank: Brigadier General
Date of Death: 21/08/1915
Age: 50
Regiment/Service: General Staff
Commanding 2nd (South Midland) Mounted Brigade.
Awards: K P, M V O

Grave Reference: Spec. Mem. E. 3.
Cemetery: GREEN HILL CEMETERY

Additional Information:
5th Earl of Longford. Born at Dublin. Son of William Lygon Pakenham, 4th Earl of Longford; husband of Mary Julia, Countess of Longford, of North Aston Hall, Oxford.
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For Pakenham:

Arms: Quarterly Or and Gules, in the first quarter an eagle displayed vert.

Crest: Out of a mural crown Or an eagle displayed Gules.

There are three other quartered arms in Burke, for Cuff, Aungier and Boyle.

Burke’s Peerage, 107th edition, pub 2003, Longford article, vol 2, p. 2389.

Their marriage index:

First name(s) MARY J
Last name CHILD-VILLIERS
Marriage quarter 4
Marriage year 1899
MarriageFinder™ MARY J CHILD-VILLIERS married one of these people
Thomas P Longford, Thomas Earl Pakenham, William Hutchinson, Olive Hopkins
District St. George Hanover Square
County London
Country England
Volume 1A
Page 902
Record set England & Wales Marriages 1837-2005
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